This invention relates to electronically commutated DC motors [ECDC motors], and more particularly, to low cost ECDC motors having inherent commutating energy suppression with over-voltage protection.
Although ECDC motors have longer life, higher operating speeds, lower maintenance requirements and lower electromagnetic interference than the industry standard DC brush motor, the ECDC motors have remained relatively more expensive then the DC brush motor. In order to make the ECDC motor more competitive with its industry rival it has become necessary to reduce its cost while maintaining its advantageous qualities.
The use of power transistors at or near their rated capacity could reduce cost by permitting the choice of lower cost power transistors, but this leaves little margin for error in the event of a voltage surge exceeding the voltage rating of the transistor. The use of higher voltage rated transistors has two disadvantages. First, it increases the cost of the transistors. Second, in the case of field effect transistors (FET's), which are often used in ECDC drives, the on resistance of the transistors increases with higher voltage rated transistors. This represents two increased costs in the system. One being the additional cost of making the motor and second, the additional power consumption used in its operation. Thus, it is very desirable to reduce the voltage rating of the power transistors used in the ECDC motor commutator circuit while taking into account such factors as temporary voltage surges in the dc power supply to the commutator circuit and commutation energy.
Another area of the ECDC motor in which cost reduction may be pursued is in the ECDC electronic drive configuration (electronic commutator configuration). A common electronic commutator is a full wave three phase drive as shown in FIG. 1. In order to reduce costs, the number of power components in the electronic commutator were reduced by converting to a half wave configuration. One such design is shown in FIG. 2. The half wave design reduced the number of power transistors needed in the electronic commutator, eliminated the need for high side drivers to control the upper power transistors, reduced the cost, and had a higher electronic drive current capability for an equivalent transistor conduction power loss. However, the half wave motor design can only operate at approximately 70% of the maximum continuous operating torque of a full wave commutator driven motor when using an equivalent rotor magnet and stator lamination design.
A second disadvantage of the half wave configuration is caused by energy stored in the magnetic field created by the stator winding when conducting current. This energy, herein referred to as the commutating energy, is imposed on the transistor when it is switched from an on to an off state. Commutation energy is typically not a problem in low power applications. However, in higher power applications, the greater commutating energy results in a high energy inductive voltage spike being imposed across the transistor. As shown in FIG. 3, this inductive spike causes the transistor to avalanche. The avalanche dissipates the commutating energy in the transistor, and at high power levels, excessive heat is created in the transistor and destroys the transistor. The result is a catastrophic failure of the commutation circuit. To suppress this avalanche, various suppression networks can be incorporated in the drive.
Two examples of suppression networks are shown in FIG. 4. These suppression networks have two undesirable traits. First, they increase cost. Second, they do not produce greater mechanical power from the motor. Thus higher power half wave drives must add an expensive snubber network which does not produce any additional mechanical power. A half wave drive design will only remain low cost if it inherently can suppress the commutating energy so that it does not cause catastrophic failure of the commutation circuit. In other words, the snubbing must be low cost or not required at all.